According to https://python.readthedocs.io/en/stable/whatsnew/3.5.html:

The SMTPServer class now advertises the 8BITMIME extension (RFC 6152) if decode_data has been set True. If the client specifies BODY=8BITMIME on the MAIL command, it is passed to SMTPServer.process_message() via the mail_options keyword. (Contributed by Milan Oberkirch and R. David Murray in bpo-21795.)

This means that process_message gets a mail_options kwarg. However, the smtpd PureProxy and MailmanProxy don't take keyword arguments, which results in an exception.

One way to trigger this is to run a debug mailserver and send a mail to it:

$ python3 -m smtpd -n
error: uncaptured python exception, closing channel <__main__.SMTPChannel connected ('::1', 52007, 0, 0) at 0x10e7eddd8> (<class 'TypeError'>:process_message() got an unexpected keyword argument 'mail_options' [/usr/local/Cellar/python/3.7.2_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncore.py|read|83] [/usr/local/Cellar/python/3.7.2_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncore.py|handle_read_event|422] [/usr/local/Cellar/python/3.7.2_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asynchat.py|handle_read|171] [/usr/local/Cellar/python/3.7.2_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/smtpd.py|found_terminator|386])